birch borer is a native insect which occurs throughout the range
of birch in North America. It is a serious pest of ornamental birch
birch borer is a slender, olive to copper-bronze colored beetle,
6-12 mm (1/4-1/2 inch) long. Females are slightly larger than males.
The larvae are white, slender, and legless with a light-brown head
capsule that is retracted somewhat into a wide, flattened first-thoracic
segment. The remaining body segments are smaller and ribbon-like
in appearance. Two brown, hardened, pincer-like structures are located
at the tip of the abdomen. Mature larvae may be up to 12 mm (1/2
emergence hole. Photo by R.A. Casagande.
birch borer attacks paper (canoe), European white (especially cutleaf
variety), gray, yellow, and other birches, as well as poplar, cottonwood
and willow. The larva bores in the trunk and larger limbs, often
girdling them. Trees weakened by drought or injured are most susceptible
injury is caused by larval tunneling in the inner bark or cambium.
The girdling of the trunk or branches interrupts sap flow downward
to the roots and destroys the tree's cambium tissue. The interruption
and subsequent accumulation of sap flow above larval tunnels often
causes characteristic swollen bands or vein-like ridges in trunks
and affected branches. The galleries range to approximately 1.5
meters (5 feet) in length and are packed with a dark brown sawdust-like
sign of bronze birch borer attack is usually a die-back of the uppermost
branches, followed by a gradual decline and eventual death of the
entire tree in two to three years. The presence of "D" shaped adult-emergence
holes in the tree trunk is a positive sign of borer activity. Attack
usually begins in crown branches 19 to 25 mm (../images/4 to 1 inch) in
diameter, progressing downward year after year. In healthy, vigorous
trees, the larvae do not complete development and the galleries
to emerge through "D" shaped holes in the bark in early to mid-June
and may continue for five to six weeks. They feed on the foliage
of alder, aspen, birch, willow or poplar for at least a week before
beginning to lay eggs in bark crevices or beneath loose bark. Like
other flatheaded borers, the bronze birch borer is attracted to
the sunny side of trees for feeding and egg laying (oviposition).
Areas of recent injury, mechanical or other, appear most attractive
to the borer. Females lay up to 76 eggs, which hatch in two weeks.
The larvae bore into the inner bark (phloem) and cambium to feed.
Mature larvae construct oblong cells in the thick bark or wood in
the fall and pupate the following spring. There is only one generation
Avoid planting susceptible (white-barked) birch species. European
white birch and its cutleaf cultivar are most susceptible to attack.
River birch, Betula nigra, has salmon-pink bark and is resistant
to the bronze birch borer. 'Heritage' is a relatively white-barked
cultivar of river birch which is readily available in clump form
and as single stems. The U.S. National Arboretum and others have
breeding programs to develop resistant birches with desirable landscape
selection for planting birch trees often contributes to the borer
attack and death of the trees. Under natural conditions, birch grows
best in cool, moist, shaded situations. It is not adapted to open,
sunny, exposed locations, such as the middle of a large, open yard
or the exposed south or west side of a building. Trees planted in
such sites will lose vigor and become weakened, increasing their
susceptibility to borer establishment. Adult borers also prefer
to lay eggs on trees in full sunlight.
It is possible
to prolong the life of susceptible trees by reducing stress. Deep
watering may be helpful during dry periods. Trunk injury by lawn
mowers, etc. can also weaken the tree and provides an attractive
bronze birch borer oviposition site. Mulching around the tree base
eliminates the need for close mowing and potential trunk injury
and retains soil moisture.
borer-infested firewood should be burned before May to stop adults
Chemical control is difficult once larvae have bored into the trunk.
Use foliar and bark insecticide sprays to control egg-laying adults
or newly-hatched larvae before they enter the bark. Apply the first
spray thoroughly to the bark and foliage in early June when Kousa
dogwood blooms. Since the adult flight period may last for six weeks
or more, two additional sprays at 3-week intervals are suggested.
Most effective insecticides for the bronze birch borer are restricted
to professional applicators. Homeowners may find permethrin available
at some garden centers.
from the University of Vermont Cooperative Extension, 1999